Tuning Aggregation Behavior of Polymer Donor viaMolecular‐Weight Control for Achieving 17.1% Efficiency Inverted Polymer Solar Cells

Abstract

Main observation and conclusionRecently, the polymer solar cells (PSCs) based on the PM6 and small molecular acceptor (SMA) Y6 have attracted considerable attention in this community for their outstanding photovoltaic performance. As well known, the molecular weight of polymer has dramatic impact on the blend morphology and photovoltaic performance in the polymer solar cells, especially for those polymers with temperature‐dependent aggregation (TDA) behavior. In this work, a series of PM6 polymers with different weight‐average molecular weights (Mw = 41, 67, 74, 84 and 114 kDa) were synthesized, and the effects of the Mw on the morphology and photovoltaic performance were systematically investigated. Notably, molecular weight induces dramatic influences on the aggregation behaviors of the polymers and their corresponding blend morphology, which were explored by the temperature‐dependent UV‐vis absorptions (TD‐Abs), grazing incidence wide‐angle X‐ray scattering (GIWAXS) and resonant soft X‐ray scattering (R‐SoXS) measurements. Increasing Mw promoted the strong face‐on molecular packing and reduced the blend film domain sizes for enhanced donor‐acceptor interfacial areas, resulting in increased short‐circuit current densities (Jsc) and fill factor (FF). However, polymer donor with high Mw value processed stronger tendency of forming excessive pure donor aggregate, which hampered forming the intermixed phase for efficient charge transport, resulting in the inferior FF. Therefore, the optimized OSCs based on PM6 with moderate molecular weight (Mw = 74 kDa) and Y6 exhibited suitable size and purity of the domains, facilitating well‐balanced charge transport, reducing trap‐assisted recombination and leading to significant enhancement of photovoltaic performance with the optimal PCE of 17.1%. This work indicates that proper and precisely tuning the molecular weight of polymer donor is critical to enhance the photovoltaic performance in PSCs.